The present disclosure relates generally to a capacitive touch sensing system. More particularly, the present disclosure relates to a capacitive touch sensing system using switched capacitor signal processing.
Capacitive touch sensors are often used in combination with a display such as a liquid crystal display (LCD) to provide a user interface for an electronic device such as a smart phone, tablet computer or other portable device. The LCD may be used to provide text and graphics information to a user. The capacitive touch sensor may be used to sense the user's touch interaction with the device, including sensing motions such as taps, swipes and rotations of a finger, stylus or other object across the surface of the touch sensor. In other applications, the touch sensor is used alone, without an LCD or other display to receive user input.
Design requirements for capacitive touch sensors include low cost and immunity to noise and other electrical interference. The capacitive touch sensor tends to operate in an environment with substantial electrical interference. In a device such as a smartphone, the adjacent LCD or other display is driven by signals that may be noisy. Other circuits of the device may be sources of interference as well. Immunity to such interference is an important design goal. Since smart phones and other devices incorporating a capacitive touch sensor are typically commodity products with relatively low profit margins, minimizing component cost and assembly cost is also important.
As devices are miniaturized and become more highly integrated, in some cases it is desirable to integrate capacitive touch sensor circuitry with display driver circuitry that generates the signals necessary to drive the LCD or other display. The sensor and driver circuitry may even be integrated in the same integrated circuit or integrated adjacently. As a consequence, the touch sensors will be close to the display pixels and will pick up more noise from the display. In a highly integrated embodiment, the touch sensors and display are integrated in a single assembly. In this case, the touch sensors will be on the LCD substrate, away from the cover glass of the sensor display assembly and thus more distant from the finger, stylus or other source of signal. Thus, the signal to noise ratio will be relatively small when noise is greater than in conventional embodiments. In such systems, it is imperative to increase the gain and hence sensitivity to be able to detect small changes in sensor capacitance. However due to large background capacitance it is not possible to do so without clipping in conventional touch sensing systems.
Conventional capacitive touch sensors make use of analog to digital technology to sense a touch or other interaction with the touch sensor. The analog to digital input circuits are typically followed by extensive digital signal processing technology. These circuits operate to perform the necessary capacitive sensing and noise filtering and other processes. However, these circuits typically require large, expensive blocks of circuitry on one or more integrated circuits. The cost of components is generally proportional to the integrated circuit area required for implementation. The cost of analog to digital components, plus digital signal processing components, can greatly add to the component cost and assembly cost of a finished product. Secondly, if the signal processing is done in digital domain, any signal below least significant bit of the analog to digital converter is lost and cannot be recovered. Increasing resolution of analog to digital converters used comes at the cost of silicon area.
Accordingly, there is a need for a capacitive touch sensor providing reduced cost, higher sensitivity and improved immunity to noise and other electrical interference.